94 INVERTEBRATE PHYSIOLOGY 



of jumping. So great is the power developed during a twitch due to a single 

 "fast"-fiber action that we can say at once that the F axon is used only in 

 jumping. All ordinary movements must utilize the Si axon alone. Ten per 

 cent of the available muscle fibers can produce a peak tension of 80 gm. in 

 the muscle, representing a thrust of 2 gm. at the foot. This is more than 

 adequate to shift the locust, itself weighing barely 2 gm., and 10% is the 

 total number of fibers innervated by the Sn, end-plate potential-type end- 

 ings. What then is the value of the Sia slow depolarizing endings? Do 

 these endings represent some sort of evolutionary degeneration? This is 

 not as fantastic as it may seem. The jumping muscles have been evolved 

 from muscles resembling the prothoracic and mesothoracic extensor tibiae 

 muscles where the "slow" system utilizes 2/5 of the fibers. Activation 

 of 2/5 of the jumping muscles produces considerable tension, enough to 

 give a small leap if developed rapidly, so a still smaller proportion of the 

 muscles would be quite adequate for the "slow" system. Only 1/5 of the 

 tension can actually be produced at all rapidly by the Si axon, since the 

 Sia fibers are activated so slowly ; an efi^ective reduction in innervation 

 has therefore been achieved. The metathoracic extensor tibiae is an im- 

 portant tonic muscle, unlike the prothoracic and mesothoracic muscles ; it 

 supports the climbing and vertical resting locust and raises the abdomen 

 during walking. The S^a type ending is probably well-suited to the tonic 

 control of the muscle. 



In making these deductions we must realize that the evolution of the 

 powerful jumping apparatus required not simply an enlargement of the 

 limb and a habit of tucking the tibia under the femur. It was also necessary 

 to develop compatible neuromuscular mechanisms from the existing ones, 

 and the much more subtle but equally important problem of altering the 

 central nervous synapses appropriately had to be overcome at the same 

 time. The requirements of tonic and slow activity had to be met at all 

 stages and effected by a muscle which acquired the functions of its an- 

 tagonist whilst increasing its power by 20 times. 



The Natural Control of Insect Muscle 



The study of neuromuscular mechanisms is only a part of the much 

 broader field of the whole functioning of the motor apparatus, itself a 

 branch of the physiological study of animal behavior. Consequently, if we 

 can advance from a purely descriptive study of the events associated with 

 transmission to a study of the natural functioning of the muscles, we enter 

 in effect the realm of the central nervous system and make contact with 

 the automatic control systems associated with proprioceptive feedback. 



With a view to obtaining information about the natural functioning of 

 locust muscles I have developed a technique which I shall describe briefly. 



